1. Field of the Invention
The present invention relates to an image-acquisition optical system having a power varying function, and more particularly, relates to an image-acquisition optical system used in an endoscope objective lens that is capable of magnified observation or a photographic lens that is capable of macro photography, such as the lens of a digital camera, video camera, mobile-phone camera, or the like.
This application is based on Japanese Patent Application No. 2006-092465, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years there has been an increasing demand in the field of medical endoscopes for image-acquisition optical systems that are capable of magnified observation for precise diagnosis of lesions. Conventional examples of such image-acquisition optical systems for magnifying endoscopes are disclosed in Japanese Examined Patent Application, Publication No. SHO-61-044283, Japanese Unexamined Patent Application, Publication No. HEI-06-317744, Japanese Unexamined Patent Application, Publication No. HEI-11-316339, and Japanese Unexamined Patent Application, Publication No. 2000-267002.
Furthermore, in the field of medical endoscopes, in order to improve the diagnosis precision, there are demands to increase the definition of endoscope images. To obtain such endoscope images, image-acquisition devices having more pixels than conventional devices have begun to be used. Therefore, high-performance image-acquisition optical systems that are compatible with such image-acquisition devices having more pixels are essential.
On the other hand, there are also increasing demands for ultra-high-magnification observation optical systems which can perform magnified observation of biological tissue in lesions etc. under an endoscope to a level equivalent to that achievable under a microscope (hereinafter referred to as “microscopic observation” for short).
Examples of image-acquisition optical systems for endoscopes which are capable of such microscopic observation are disclosed, for example, in Japanese Unexamined Patent Application, Publication No. 2004-313769 and Japanese Unexamined Patent Application, Publication No. 2004-313772.
Conventionally, in order to perform microscopic observation of biological tissue, after identifying a lesion with an endoscope, some cells are excised and a biopsy is performed by microscope examination. However, the approaches in the above-described Japanese Unexamined Patent Application, No. 2004-313769 and Japanese Unexamined Patent Application, No. 2004-313772 have the merit that it is possible to both carry out endoscope examination while at the same time examining cells inside the body.
High magnification, for example, for macro photography, and compactness are also desirable in optical systems for consumer devices, such as digital cameras and video cameras.
However, in the endoscope image-acquisition optical systems described in Japanese Examined Patent Application, Publication No. SHO-61-044283, Japanese Unexamined Patent Application, Publication No. HEI-06-317744, Japanese Unexamined Patent Application, Publication No. HEI-11-316339, and Japanese Unexamined Patent Application, Publication No. 2000-267002, the observation magnification during close-up magnification (at the telephoto end) is not high enough to allow microscopic observation.
Furthermore, the endoscope image-acquisition optical systems for microscopic observation described in the above-mentioned Japanese Unexamined Patent Application, Publication No. 2004-313769 and Japanese Unexamined Patent Application, Publication No. 2004-313772 are only capable of close-up magnified observation at a fixed magnification. Therefore, an image-acquisition optical system having normal observation magnification and an image-acquisition optical system for performing microscopic observation must be separately provided. Accordingly, when using the endoscope image-acquisition optical systems for microscopic observation described in the above-mentioned Japanese Unexamined Patent Application, Publication No. 2004-313769 and Japanese Unexamined Patent Application, Publication No. 2004-313772 to perform microscopic observation, a probe provided in the image-acquisition optical system for microscopic observation must be guided through an instrument-inserting channel of the endoscope to the subject to be acquired.
As a result, with the endoscope image-acquisition optical systems for microscopic observation described in the above-mentioned Japanese Unexamined Patent Application, Publication No. 2004-313769 and Japanese Unexamined Patent Application, Publication No. 2004-313772, some parallax occurs between the viewing field of the endoscope and the viewing field of the microscope, which makes it difficult to identify the region of the subject to be microscopically observed. The region observed during microscopic observation with the endoscope image-acquisition optical system for microscopic observation, however, is only an extremely narrow region on the order of several tens of square micrometers to several hundred square micrometers. Also, it is difficult to completely immobilize the tip of the endoscope at the site to be observed due to the effect of pulsing and so forth in the subject body. Therefore, it is difficult to identify an examination site with such an optical system having parallax.
The probe provided in the image-acquisition optical system for microscopic observation is used while being pressed against the examination site. However, because this probe has a diameter small enough to pass through the instrument-inserting channel, the area pressing against the examination site is small, which makes it difficult to secure to the examination site. Thus, the image-acquisition optical system for microscopic observation is used while being inserted inside the body through the instrument-passing channel, and therefore, it has the drawback that it is difficult to securely fix the optical system for microscopic observation during observation.
In order to stabilize the endoscope during microscopic observation, it is also possible to dispose the image-acquisition optical system having a normal observation magnification and an image-acquisition optical system for microscopic observation in a tip-inserting portion. In this case, however, the problem of parallax between the viewing field during normal observation and the viewing field during microscopic observation cannot be overcome. Furthermore, in this case, the tip-inserting portion of the endoscope is equivalent to a device in which two scopes of the same size are combined, resulting in a large endoscope. In particular, because the outer diameter of the tip-inserting portion of the endoscope is large, the burden placed on the patient receiving the endoscopic examination is significant, which is undesirable.